Chlorine isotope ratios record magmatic brine assimilation during rhyolite genesis

Magmatic volatile phases within crustal silicic magma domains influence key volcanic processes such as the build up to eruptions and formation of magmatic-hydrothermal ore deposits. However, the extent and nature of fluid-melt interaction in such environments is poorly understood, as geochemical signals in volcanic rocks originating from pre-eruptive volatile processes are commonly overprinted by syn-eruptive degassing. Here, we use δ37Cl as a conservative tracer of brine-melt interaction on a broad suite of silicic volcanic rocks from Iceland. We find that the δ37Cl values of silicic rocks are systematically shifted to more negative values compared to associated basalts and intermediate rocks by up to 2.9 . These large shifts cannot be explained by well known processes inherent to silicic magma genesis, including crustal assimilation, mineral-melt fractionation and syn-eruptive degassing. Instead, we show that low δ37Cl values in silicic rocks can be attributed to assimilation of magmatic brines that are formed and stored in long lived crustal magma mushes. Our results indicate that magmatic brine assimilation is a fundamental, but previously unrecognised part of rhyolite genesis.